This invention relates to well boring apparatus, and in particular to a keyseat wiper or reamer which eliminates or reduces troublesome curves or doglegs occurring in the drilled bore.
During the boring of a drilled well, and in particular oil and gas wells where many strata of rock are encountered, slots are often formed in the well bore during the drilling operation or as the string of drill pipe is withdrawn from the well. Because the drill pipe exerts tremendous force on a curved wall of the bore, the pipe tends to carve a groove into the wall, known as a "keyseat". Since the drill pipe is typically of smaller diameter than the diameter of the drill bit and of the drill collar or collars, removal of the pipe when a keyseat has been formed often causes a drill collar or the drill bit to lodge in the keyseat. At best, efforts to remove the lodged collar or drill bit cause a delay in the drilling operation or a costly process of fishing for the lodged bit.
The prior art has recognized the inherent problems of keyseats, and has developed many different types of tools for eliminating keyseats. For example, U.S. Pat. No. 2,904,313 discloses a reamer for a keyseat which comprises a plurality of cutting elements welded to the reamer body in a spiral fashion. The cutting elements include hardened or formed edges for reaming the keyseat.
Another prior art keyseat reamer is disclosed in U.S. Pat. No. 3,420,323. The reamer comprises a series of cutting elements attached either axially or spirally to the cylindrical body of the reaming tool. The cutting elements comprise outstanding ribs which are dressed with a hardened cutting material.
One problem with prior art keyseat reaming tools is strength. Those tools in which the keyseat reamer is merely a sleeve fitted on a portion of the drill pipe are prone to breakage. Also, because the cutting elements of the prior art extend beyond the outer diameter of the drill pipe, if the keyseat reamer comes into contact with the metal casing for the well, the reamer can cut into the casing, weakening the casing and creating the danger of a "blowout" in wells which are subjected to high pressure.